Acrylic acid is a raw material monomer for polyacrylic acid or acrylic acid copolymers. Because of an increase in the amount of a water-absorption resin (sodium polyacrylate) used, the amount of acrylic acid produced has been increased. Acrylic acid is generally produced by synthesizing acrolein from propylene that is a raw material derived from petroleum and then by converting this acrolein to acrylic acid by catalytic gas-phase oxidation (Patent Document 1). However, there is a fear that such a petroleum-derived raw material will be depleted in the future. Because of such a fear, studies have been conducted for the purpose of obtaining an unsaturated carboxylic acid from biomass, and for instance, a method for synthesizing an unsaturated carboxylic acid or an ester thereof from an ammonium salt of hydroxycarboxylic acid has been disclosed (Patent Document 2). However, the method described in the Patent Document 2 comprises complicated steps, such as the necessity of separating the ammonium salt of hydroxycarboxylic acid into a hydroxycarboxylic acid and a non-aqueous ammonium cation-containing exchange resin.
Hence, a method for synthesizing an unsaturated carboxylic acid or a derivative thereof from a biomass-derived hydroxycarboxylic acid or a derivative thereof by a dehydration reaction using hydroxyapatite (Ca10(PO4)6(OH)2) or Sr10(PO4)6(OH)2 as a catalyst has been proposed (Patent Document 3). Nevertheless, when acrylic acid is synthesized from lactic acid using the aforementioned hydroxyapatite, the yield of the acrylic acid is only approximately 50% to 70%, and when Sr10(PO4)6(OH)2, in which Ca in hydroxyapatite is replaced with Sr, is used as a catalyst, the yield of the acrylic acid is reduced to approximately 30%. Thus, it has been desired to develop a method for synthesizing an unsaturated carboxylic acid such as acrylic acid, or an ester thereof, at a higher yield.